In optical and magneto-optical recording systems, information is recorded as marks created in information storage tracks formed on a recording element which can be read out by means of a focused optical beam. The recording element conventionally takes the form of a circular disk with concentric or spiral data tracks, although other forms of recording elements are possible. For this discussion, the recording element will be assumed to be in conventional disk format.
In the case of optical recording, the information can be stored on the media in the data tracks in the form of minute surface features, such as pits, which produce intensity variations in the optical readout beam which can be detected and decoded by suitable sensor electronics. In the case of magneto-optical recording, the information is stored as reversed magnetic domains in a magnetic layer along the data tracks which cause changes in polarization of the readout beam. These polarization changes are sensed by photo detectors preceded by suitably aligned polarizers. In this discussion, the term "optical", in relation to the storage medium, will be used interchangeably herein to refer to both purely optical and magneto-optical information storage systems.
It is well known in the operation of optical information storage systems that it is necessary to maintain the scanning optical beam in sharp focus on the recording element and also to precisely control the beam's lateral position to assure that a desired data track is being followed. To meet these requirements, conventional optical recording systems incorporate independent servo sensor systems to produce focus and tracking error signals based on the detection of certain characteristics of the readout beam. Typically, the readout beam returned from the disk contains diffraction order beams scattered by the grooved profile of the data tracks on the disk. It is known to use differential detection of the overlap regions between the plus and minus first order diffraction beams with the zeroth order beam in the readout beam to derive the tracking error signal.
A variety of detection techniques are also known for deriving the focus error signal, examples of which are knife edge detection, astigmatic beam detection, and spot-size detection. Of the various known techniques, it can be shown that spot-size detection has the advantage of generating the least amount of crosstalk between the tracking error signal and the focus error signal. Conventional spot-size detection is illustrated in FIG. 1 wherein a circular readout beam 10 is focused near a planar detector array 16 which is comprised of six detection elements A-F. Each of the detector elements are coupled through separate preamplifiers to sum and difference amplifiers (not shown) to derive the appropriate focus and tracking error signals. The signal (A+C+D+F)-(B+E) is an indication of spot-size and is therefore a good focus error signal since the beam spot size grows or shrinks as the disk moves ahead and behind the focal point of the incident optical beam.
Beam 10 includes a circular-shaped zeroth order beam 13 and plus and minus first order diffraction beams 14 and 15 which form overlap lobes 16 and 17 with the zeroth order beam 13. The push-pull tracking error signal is derived from the signal (A+B+C)-(D+E+F). Because the beam displacement directions for tracking and focus are orthogonal, there is little crosstalk between these signals. However, for this detection arrangement, six separate preamplifiers are required. Also, an undesired offset in the focus error signal is caused when the beam moves from areas of tracks and non-track or mirrored areas on the disk, where the diffraction order beams disappear. A further difficulty with the spot-size detector of FIG. 1 arises from the fact that the gain of the focus error signal is relatively low. Although the gain of the focus error signal can be improved by moving the detector near the beam focus where the spot is smaller, this has the disadvantage of making the tracking signal very sensitive to very small beam movements caused by physical changes in the disk drive such as misalignment resulting from thermal changes, vibration etc..
U.S. Pat. No. 4,841,509, by Kamisada et al., issued Dec. 3, 1986, shows a modified spot-size focus detection arrangement which reduces the effect on tracking offset which occurs with these small beam movements. In this patent, a pair of four element spot-size detectors are positioned in separate paths of the readout beam on opposite sides of the beam focal point. In each path the beam is converted from a circular to an elliptical shape with the major axis of the beams aligned with the central elements of each detector for detection of the tracking error signals. As the disclosure indicates, the expansion of the beam in the tracking error direction on the detector elements reduces the sensitivity of the tracking error signal to the movement of the beam caused by physical changes in the disk drive system. The Kamisada disclosure, however, requires two separate detectors with the focus error signal being derived from the outside detector elements, thus requiring as many as eight preamplifiers with consequent additional cost and complexities in the design of the servo detection system.
It is, therefore, an object of the present invention to provide an improved focus and tracking error detection apparatus based on spot-size focus detection that utilizes a single detector array to achieve the desired focus and tracking error signals.
It is a further object of the invention to provide a focus and tracking error detection apparatus utilizing a single detector array that minimizes focus offset caused by transition of the optical beam between track and non-track areas on the disk.